Refinery off gases can provide a source of hydrogen which can be used by others or used in the refinery itself. The off gases comprise H2 as well as hydrocarbons which can be converted to H2. The off gases from various different processes in the refinery may be combined to form a refinery fuel gas feed (RFG feed). The RFG feed can be used as feed for a steam methane reformer to produce H2 required, for example, for refinery hydrocracking and desulfurization units.
While the RFG feed is potentially a rich source of hydrogen, its utilization is not without problems. The RFG feed, being a mixture of off gases from a number of different processes, comprises a wide variety of constituents, some of which are detrimental to the steam methane reforming process. Such constituents include olefins such as ethylene, propylene, butenes and other alkenes as well as sulfur compounds such as mercaptans, sulfides, COS and thiophenes. Particular examples of these sulfur compounds include H2S, COS, methyl mercaptan, ethyl mercaptan, dimethyl sulfide and thiophene. Before such a gas mixture can be processed in a steam methane reformer, the various olefins must be hydrogenated to avoid coking of the steam methane reformer catalyst, and the sulfur compounds must be removed to avoid catalyst poisoning.
Another difficulty in the utilization of RFG feed arises because both the composition and available volume of the feed may vary substantially over relatively short time periods. For example, the concentration of olefins and hydrogen in an RFG feed may vary significantly during daily operation.
Prior art methods for pre-treating RFG feeds to steam methane reforming involve hydrogenating the olefins by reacting the RFG feed with hydrogen in an adiabatic reactor containing a catalyst comprising a support, such as alumina, impregnated with metal compounds, such as Co, Mo, and Ni types of hydrogenation catalysts. Organic sulfur compounds are also hydrogenated in the presence of these catalysts to produce H2S, which may then be removed by passing the processed feed through a bed of zinc oxide. The resultant gas stream may then be processed in a steam methane reactor.
One disadvantage of known prior art processes is that they cannot readily handle concentrations of olefins greater than 4 to 6 mole %, and cannot adapt to the full potential variability of the feed gas composition. This is due to the highly exothermic nature of the olefin hydrogenation reaction combined with the relatively high reactor inlet temperatures necessary to initiate hydrogenation in the presence of the catalysts. Temperatures in the hydrogenation reactor are limited to a maximum of 398-427° C. to prevent hydrocarbon cracking, which is undesirable. With typical inlet temperatures from 249° C. to 302° C. (depending upon the choice of catalyst), the maximum temperature limit of 398-427° C. imposes a limit on the olefin concentration from 4 to 6 mole % for an adiabatic reactor.
Prior art methods which address this problem of high olefin concentration include blending natural gas with the RFG feed as necessary in order to dilute the olefin concentration to an acceptable level, or recycling some of the outlet gas from the reactor to dilute the RFG feed. The use of an isothermal reactor upstream of an adiabatic reactor in series has also been considered. These solutions tend to restrict RFG utilization, or they are expensive, consume more power and require more complicated equipment and controls as well as larger capacity equipment. There is clearly a need for a hydrogenation process that can handle concentrations of olefins in an adiabatic reactor higher than 4 to 6% without exceeding the maximum temperature limitations.